The present invention relates to a keyboard, such as the keyboard for a musical instrument, and in particular to an electrical system and method for calibrating the keyboard to provide for uniformity of response.
In prior art electronic keyboards, such as those used in musical instruments such as organs and electronic pianos, an electric or electronic switch is closed when the key has been depressed a certain degree to thereby indicate to the electronic circuitry that the key has been struck. Some keyboards are touch responsive whereby the output from the keyboard varies depending on the velocity with which the key is struck. One typically used system for determining key velocity is to time the transition of the key from its undepressed state to a fully depressed state wherein the amount of time elapsed is inversely proportional to the velocity with which the key is struck. In other prior art keyboards, piezoelectric devices or other force transducers are used.
In many prior art keyboards, both those used in electronic musical instruments and in other applications, it is important to provide an output signal which corresponds to the level of key depression that the performer or keyboard operator determines is the fully depressed position of the key. The performer normally detects this through a tactile sensation, such as when the key bottoms out against a stop. However, the mechanical adjustment of keyboards and keys within a keyboard can vary substantially so that the signal indicating full key depression may occur before or after the point at which the performer or operator assumes the key has been fully depressed.
In the aforementioned Pat. No. 4,558,623, which application is incorporated herein by reference, there is disclosed a prior art keyboard system wherein the position of the key is detected by a movable pickup that moves within an electrostatic field formed between a pair of stationary electrodes. The pickup never contacts either of the electrodes, but the voltage impressed on the pickup varies as a function of the pickup within the electric field so that as the key is depressed and the pickup is moved within the field, the voltage impressed thereon changes with position. In that particular keyboard, the voltages impressed on the stationary contacts are substantially of equal amplitude, but 180.degree. out of phase so that at the midpoint of the pickup between the stationary electrode, the voltage impressed thereon is substantially zero. The system is designed to be responsive to this critical zero voltage level and provide an output signal that indicates the key is "fully closed". Further depression of the key beyond this point produces a signal of opposite polarity, which is sensed and used by the system as an aftertouch control. The performer determines that the key has been fully depressed through a tactile sensation when the key impacts against a resilient stop, such as a Poron washer or the like.
In a keyboard such as this, it is important to generate the critical output voltage at exactly the point where the key contacts the resilient stop, or at least the performer perceives the key as contacting the resilient stop, because the time between the beginning of key closure and the sensing of the critical voltage level informs the system of the velocity with which the key is struck. Any further depression of the key beyond this point is sensed as aftertouch, which provides a variable voltage that can be processed by the system to generate tremelo or other musical effects common to aftertouch manipulation of the key.
A problem which many prior art keyboards is that the mechanical switch structure is not uniform from key to key, so that when two keys are depressed to what the performer perceives as their fully depressed positions, the trigger or switch closure signals are not produced at the same time. In the keyboard system of Pat. No. 4,558,623, for example, if one of the stationary contacts or the movable contact is bent or otherwise not perfectly mechanically adjusted, the zero voltage level will be reached at a point other than what the performer perceives as full key closure. This results in non-uniform switch response and unpredictable aftertouch control.